Director Rachel Talalay creates some particularly macabre images in the scenes of the creatures being mowed down by the defenders' rifles and then fastened to wooden crosses like scarecrows while children look on, inured to the sight.
In her final performance for the show, Mackie is as good as she's been all season, effectively capturing Bill's growing resignation to the fact that even the Doctor has no hope of reversing her condition.
On the one hand, Moffat has once again (as with Clara in 2015's “Hell Bent”) arranged for a companion to suffer a dramatic fate and then abruptly reversed it; on the other, the ingenious callback to the end of “The Pilot,” as Heather describes how she found Bill through her tears (the otherwise unexplained tears seen earlier emanating from Bill in her Cyber form), creates a story arc for Bill that helps to bind the entire season into a satisfying whole.
Through the snow appears none other than the original Doctor, heading toward his own regeneration in the last moments of 1966's “The Tenth Planet.” David Bradley, who brilliantly portrayed William Hartnell in the docudrama produced for Doctor Who's 50th anniversary in 2013, An Adventure in Space and Time, brings the first Doctor back to life as, in his final moments, he comes face to face with his own later self.
Classic Doctor Who Recommendation: For another story featuring a pair of male and female Time Lords—the Master and the Rani—who bicker and scheme against each other as much as against the Doctor, see 1985's “The Mark of the Rani,” starring Colin Baker and Nicola Bryant, with Anthony Ainley and Kate O'Mara as the two Time Lords.